Servlet简介

标签：HttpServletRequest 简介 resp req HttpServletResponse IOException Servlet void

1. 把实现了servlet接口的Java程序叫做servlet

编写一个类，实现servlet接口
把开发好的Java类部署到web服务器中

2. HelloServlet

构建一个普通的maven项目，删掉src目录，就可以建立更多的module；这个空工程就是maven主工程
pom.xml中添加依赖

<dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
        </dependency>
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>jsp-api</artifactId>
            <version>2.2</version>
        </dependency>

可以在此路径下找到对应的jar包
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将web.xml换成最新的

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">


</web-app>

创建并标注，将maven的结构搭建完整
编写一个普通类，实现servlet接口，这里直接继承HttpServlet

public class HelloServlet extends HttpServlet {

    //由于get或者post只是请求实现的不同的方法，可以相互调用，业务逻辑都一样

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

//        ServletOutputStream sos = resp.getOutputStream();
        PrintWriter writer = resp.getWriter();//响应流
        writer.println("hello,Servlet");

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

web.xml中编写servlet的映射
- 为什么需要映射：我们写的Java程序，但是要通过浏览访问，而浏览器需要连接web服务器，所以我们需要在web服务中注册我们写的Servlet，还需要给他一个浏览器能够访问的路径。

<servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.gy.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

配置tomcat local，并测试
tips
以HTML输出

resp.setContentType("text/html;charset=utf-8");
PrintWriter writer = resp.getWriter();//响应流
writer.println("<h1>404</h1>");

3. Servlet原理

Servlet是由web服务器调用，web服务器在收到浏览器请求之后，会调用相关API对请求处理。
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一个请求（Servlet）可以指定通用映射路径

<!--默认请求路径-->
<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>/*</url-pattern>
</servlet-mapping>

指定一些后缀或者前缀等

<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>*.do</url-pattern>
</servlet-mapping>

优先级问题
指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求；

4. ServletContext

web容器在启动的时候，它会为每个web程序都创建一个ServletContext对象，它代表了当前的web应用。
共享数据
- 存数据的Servlet

public class setData extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("hello");

        ServletContext servletContext = this.getServletContext();

        String username = "爱丽丝";//数据
        servletContext.setAttribute("username",username);//将一个数据保存在ServletContext中  名为：username  值为 爱丽丝
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

读取数据的Servlet


public class GetData extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        String username = (String) context.getAttribute("username");

        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html");
        PrintWriter writer = resp.getWriter();
        writer.write(username);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

获取初始化参数
web.xml

<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取上下文对象
        ServletContext context = this.getServletContext();
        //获取初始参数
        String url = context.getInitParameter("url");
        
        System.out.println("url: "+url);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

测试结果：页面输出url

请求转发，路径不会变

context.getRequestDispatcher("/转发的请求路径").forward(req,resp);

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读取配置文件

clean掉target，然后启动项目可以发现resource下的配置文件.properties在target的路径/WEB-INF/classes/db.properties

public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream stream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/cn/bloghut/servlet/aa.properties");
//        InputStream stream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties pt = new Properties();
        pt.load(stream);
        String username = pt.getProperty("username");
        String password = pt.getProperty("password");
        resp.getWriter().println(username+":"+password);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

maven资源导出失败问题
在pom.xml中添加下列代码

<build>
    <resources>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.xml</include>
                <include>**/*.properties</include>
            </includes>
        </resource>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
        </resource>
    </resources>
</build>

5. HttpServletResponse

web服务器接收到客户端的http请求，会针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletRespoonse；

如果要获取客户端请求过来的参数：找HttpServletRequest
如果要给客户端响应一些信息：找HttpServletResponse

下载文件
1.要获取文件的路径
2.下载的文件名是啥
3设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西
4.获取下载文件的输入流
5.创建缓冲区
6.获取OutputStream对象
7.将FileOutputStream流写入到buff缓冲区
8.使用OutputStream将缓冲区中的数据输出到客户端！

 @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String realPath = "E:\\workspace_idea1\\javaweb-servlet\\response\\src\\main\\resources\\粒子群.jpg";
        //注意这里+1的位置易出错
        String fileName = realPath.substring(realPath.lastIndexOf("\\")+1);
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
        //获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath);
        int len = 0;
        byte[] buffer = new byte[1024];
        ServletOutputStream out = resp.getOutputStream();
        while ((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }

验证码功能

前端实现
后端实现，需要用到Java图片类，生成一个图片

 @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //浏览器三秒刷新一次
        resp.setHeader("refresh","3");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//笔
        //设置图片背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.blue);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器，这个请求用图片打开
        resp.setContentType("image/jpg");

        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");
        //把图片写给浏览器
        ImageIO.write(image,"jpg",resp.getOutputStream());

    }

    //生成随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(9999)+"";
        StringBuffer sb = new StringBuffer();
        for (int i = 0;i <4-num.length();i++){
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

实现重定向
B一个web资源收到客户端A请求后，B他会通知客户端去访问另一个web资源，这个过叫重定向。

用户登录

public void sendRedirect(String location) throws IOException;

 @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        /*
        resp.setHeader("Location","/r/img");
        resp.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);//302重定向，状态码
         */
        resp.sendRedirect("/r/img");//此处为验证码Servlet的路径
    }

重定向和转发

相同点
页面都会实现跳转
不同点
请求转发的时候，url不会产生变化 307
重定向的时候，url地址栏会发生变化 302

一个DEMO
index.jsp,其中action后路径与Servlet对应

<html>
<body>
<h2>Hello World!</h2>
<%--当前项目路径--%>
<%@ page contentType="text/html; charset=UTF-8"  %>
<form action="${pageContext.request.contextPath}/login" method="get">
    用户名：<input type="text" name="username"><br>
    密码：<input type="password" name="password"><br>
    <input type="submit">
</form>
</body>
</html>

RequestTest.java

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username+":"+password);
        resp.sendRedirect("/r/success.jsp");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }

6.HttpServletRequest

HttpServletRequest代表客户端的请求，用户通过Http协议访问服务器，Http请求中的所有信息会被封装到HttpServletRequest，通过这个HttpServletRequest中的方法，获取客户端的所有信息。

获取前端传递的参数

req.getParameter

请求转发
- 转发时"/“代表的是本应用程序的根目录重定向时”/"代表的是webapps目录
- getRequestDispatcher分成两种，可以用request调用，也可以用getServletContext()调用不同的是request.getRequestDispatcher(url)的url可以是相对路径也可以是绝对路径。而this.getServletContext().getRequestDispatcher(url)的url只能是绝对路径。
  index.jsp

<%--
  Created by IntelliJ IDEA.
  User: Ray
  Date: 2022-04-24
  Time: 20:53
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
<h1>登录</h1>
    <form action="${pageContext.request.contextPath}/login" method="post">
        用户名：<input type="text" name="username"><br>
        密码：<input type="password" name="password"><br>
        爱好：
        <input type="checkbox" name="hobbies" value="girl">女孩
        <input type="checkbox" name="hobbies" value="movie">电影
        <input type="checkbox" name="hobbies" value="games">游戏
        <input type="submit">
    </form>
</body>
</html>

LoginServlet.java

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbies = req.getParameterValues("hobbies");
        System.out.println("=============");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbies));
        System.out.println("=============");
        //通过请求转发
        req.getRequestDispatcher("/success.jsp").forward(req,resp);

标签：HttpServletRequest,简介,resp,req,HttpServletResponse,IOException,Servlet,void
From： https://www.cnblogs.com/gyViolet/p/16934106.html